It is known that indole derivatives are used as units for the synthesis of active pharmaceutical ingredients. For example, 2-(2-aminopyrimidin-4-yl)-1H-indole-5-carboxylic acids or their salts are important units for the preparation of IkB kinase inhibitors (see WO 01/30774 A1):

2-(2-Aminopyrimidin-4-yl)-1H-indole-5-carboxylic acids can be prepared by classical Fischer indole synthesis starting from the corresponding 4-acetylpyrimidines (III) and 4-hydrazinobenzoic acid (II) (see scheme 1):

One disadvantage here is the severe reaction conditions which are required for a full conversion. Secondly, the products of this reaction are obtained in a mixture with the corresponding oligomers, which leads to a poor isolability, especially with regard to the filtration times. Moreover, these oligomers, owing to the low solubility of 2-(2-aminopyrimidin-4-yl)-1H-indole-5-carboxylic acids in organic solvents, can only be removed with difficulty and are entrained as an impurity in the further reactions, in some cases up to the active ingredient.